A well known misunderstanding involving High Pressure Hydraulic Ram is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal can be the real cause of cylinder drift, the physics involved are often misunderstood. Fact is, f the piston seal is entirely taken off a double-acting cylinder, the cylinder is filled with oil and the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
Within this condition, due to the unequal volume on either side in the piston, fluid pressure equalizes as well as the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move only if fluid escapes from the cylinder via the rod seal or its ports.
Exceptions for the Rule – The two main exceptions to this particular theory. The first is a double-rod cylinder (Figure 1) where volume is equal on both sides in the piston. The 2nd exception involves a lot hanging on a double-acting cylinder (Figure 2). Within this arrangement, the volume of pressurized fluid on the rod side could be accommodated on the piston side. But since the cylinder drifts, a vacuum will develop on the piston side because of unequal volumes, and depending on the weight of the load, this vacuum may eventually lead to equilibrium that arrests further drift.
This may not be the final of the cycle, but it’s important to at least grasp this theory before continuing. Notwithstanding both of these exceptions, in case a double-acting cylinder’s service ports are blocked by a closed-to-actuator spool (Figure 3), as well as the piston seal does bypass, pressure could eventually equalize on both sides from the cylinder. At this point, a hydraulic lock is effected with no further drift can happen, unless fluid is allowed to escape through the cylinder or cylinder circuit.
Loss in Effective Area – Because of the loss in effective area as a result of pressure now acting on the rod-side annulus area, the static pressure within the cylinder must increase to aid the identical load. Remember, force designed by a cylinder is a product of pressure and area. As an example, in the event the load-induced pressure on the piston side from the cylinder was 2,000 PSI and zero on the rod side once the directional control valve closed, assuming no leakage beyond the spool, the equalized pressure could be 3,000 PSI depending on the ratio of the piston and annulus areas.
New call-to-action – Now consider so what can happen if this circuit features a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes across the piston seal as well as the increasing static pressure on the piston side in the Hydraulic Press Machine Components reaches the cracking pressure from the port relief, however the cylinder will still not retract. A comparable situation can take place in circuits having a load control (counterbalance) valve installed. In this particular circuit, shown in Figure 5, the directional control valve has a float center spool (service ports A and B ready to accept tank).
As previously stated, if the piston seal leaks, unequal volumes of oil on the rod and piston sides from the cylinder indicates hydraulic lock may prevent any noticeable drift. But when again, due to the loss of effective area because of the identical pressure now acting on the piston and rod-side annulus areas, the static pressure in the cylinder must increase to back up exactly the same load.
The magnitude with this pressure increase depends on the ratio in the cylinder’s piston and annulus areas. If the rise in static pressure exceeds the set maximum load in the counterbalance valve, the valve will open allowing oil from the piston side from the cylinder to circulate towards the tank as well as the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, as the root cause in the problem in both examples is the leaking piston seal, the physics is fundamentally distinct from the general belief. And if the idea is understood, a pressure gauge can be a useful tool for establishing the cause of cylinder drift. In both of those examples, if the cylinder is drifting however, there is no equalization of pressure over the piston seal, the directional control valve or load control valve will be the supply of rldvub problem.
What exactly is the maximum pressure range for the application? Keep in mind pressures can vary greatly greatly depending on the specific job the program does. Cylinders are rated for nominal (standard) pressure and test pressure to account for variations. System pressure should never exceed the nominal rated design pressure of the cylinder.
Push or pull-or both (double acting)? The reply to this query might require Quality Hydraulic Hose in the event the hydraulic product is doing “double duty.” (Single-acting cylinders extend the piston under hydraulic pressure; double-acting cylinders extend and retract the piston under pressure.) In a push application, it is extremely vital that you size the rod diameter properly to prevent rod buckling. In a pull application, it is important to size the annulus area (piston diameter area minus the rod diameter area) correctly to maneuver the load at the rated design pressure from the cylinder.
Yongxiang Hydraulic Equipment Co., Ltd locates in Yangzhou, CN, and it covers an area of 143,500 square feet. Winning customer trust with innovation, Yongxiang has been aiming to provide customers with safe and reliable hydraulic products, services, on-time delivery, and customer satisfaction while ensuring employee safety, fostering employee relations and driving efficiency improvements.
Jiangsu Yongxiang Hydraulic Equipment Co. Ltd
Factory: Wujian Industrial Park, Jiangdu District, Yangzhou, CN
Office: 3107# No.2 Building, Global Financial Center, Wenchang East Road, Yangzhou, CN
E-mail: [email protected]